1. Field of the Invention
The present invention relates to a DC-DC converter that converts the voltage of inputted power to a different voltage.
2. Description of the Related Art
The efficiency of a DC-DC converter that converts the voltage of inputted power to a desired voltage and outputs it can be enhanced by reducing switching loss by a switching technology. In conjunction therewith, such a passive element as an inductor and a capacitor can be reduced in size by increasing the drive frequency of a switching element.
G. Hua et al. “Novel zero-voltage-transition PWM converters” IEEE transactions on Power electoronics, Vol. 9, pp. 213-219, 1994 (Non-patent Document 1), discloses a DC-DC converter that performs soft switching operation. Its circuitry is generally obtained by connecting the following auxiliary resonant circuit to a conventional boost chopper circuit comprised of a main inductor, a main switching element, a main diode, and a load: an auxiliary resonant circuit comprised of a resonant capacitor, a resonant inductor, a saturable inductor, an auxiliary switching element, and an auxiliary diode. The main switching element is caused to perform soft switching operation by the auxiliary resonant circuit.
When a series circuit of the saturable inductor and the diode is removed from the components of the auxiliary resonant circuit in the circuit described in Non-patent Document 1, the following takes place: the auxiliary diode recovers and this increases loss in the auxiliary diode and the auxiliary switching element. To avoid this, use of the series circuit of the saturable inductor and the diode is indispensable in the circuit described in Non-patent Document 1.
For the above reason, in the circuit described in Non-patent Document 1, loss in the auxiliary diode and the auxiliary switching element is suppressed by the series circuit of the saturable inductor and the diode. However, loss caused by the passage of current through the series circuit of the saturable inductor and the diode exists and thus the significant enhancement of efficiency cannot be expected. In addition, in the auxiliary resonant circuit for causing the main switching element to perform soft switching operation, two inductors, the resonant inductor and the saturable inductor, must be used as windings and this make it difficult to reduce circuit size.
It is an object of the invention to provide a small and efficient DC-DC converter that does not use a series circuit of a saturable inductor and a diode.
It is another object of the invention to provide a DC-DC converter that makes it possible to adopt an inductor of a relatively small capacity as an auxiliary inductor and thus reduce size and weight.